The present invention relates to motion control systems and, more specifically, to motion control systems incorporating multiple movers propelled along a track using a linear drive system.
Motion control systems utilizing movers and linear motors can be used in a wide variety of processes (e.g. packaging, manufacturing, and machining) and can provide an advantage over conventional conveyor belt systems with enhanced flexibility, extremely high speed movement, and mechanical simplicity. The motion control system includes a set of independently controlled “movers” each supported on a track for motion along the track. The track is made up of a number of track segments that, in turn, hold individually controllable electric coils. Successive activation of the coils establishes a moving electromagnetic field that interacts with the movers and causes the mover to travel along the track. Sensors may be spaced at fixed positions along the track and/or on the movers to provide information about the position and speed of the movers.
Each of the movers may be independently moved and positioned along the track in response to the moving electromagnetic field generated by the coils. In a typical system, the track forms a closed path over which each mover repeatedly travels. At certain positions along the track other actuators may interact with each mover. For example, the mover may be stopped at a loading station at which a first actuator places a product on the mover. The mover may then be moved along a process segment of the track where various other actuators may fill, machine, position, or otherwise interact with the product on the mover. The mover may be programmed to stop at various locations or to move at a controlled speed past each of the other actuators. After the various processes are performed, the mover may pass or stop at an unloading station at which the product is removed from the mover. The mover then completes a cycle along the closed path by returning to the loading station to receive another unit of the product.
Due to the repetitive nature of the process, each mover will typically follow the same motion profile along the length of the track. In other words, each mover will accelerate or decelerate at the same location and similarly travel at a constant speed along the same segments of the track. As is understood in the art, changing the speed of the mover typically requires higher current than operating at a constant speed. Similarly, controlling motion of a loaded mover typically requires higher current than controlling motion of an empty mover. Thus, controlling the segments of track at which each mover is accelerating or decelerating will typically experience higher currents than segments of track at which each mover travels at a constant speed. Similarly, segments of track at which the mover is loaded, or experiencing an external force resulting from another actuator, may experience higher currents than, for example, a segment during which the mover is empty and having no action performed.
As previously indicated, coils are located along the track to generate a moving electromagnetic field by which each mover is propelled along the track. Because the change in speed occurs at the same location for each mover, the coils positioned along the track at that location are required to conduct a higher current and, therefore, generate an increased magnitude electromagnetic field to effect the change of speed on the mover. Further, to avoid abrupt changes in speed, the change in speed typically occurs over multiple coils spaced adjacent to each other at the location at which the change in speed occurs. In addition, each mover may span multiple coils and, therefore, the coils across which a mover is positioned will conduct the same magnitude of current. As a result of movers spanning multiple coils and utilizing multiple adjacent coils to create a change in speed of a mover, successive coils placed adjacent to each other typically have similar levels of elevated current.
The current flowing through the coils in a linear drive system may be regulated with a power converter that includes a processing unit and a series of switching devices, such as silicon controlled rectifiers (SCRs), thyristors, or transistors, such as power metal-oxide-semiconductor field-effect transistors (MOSFETs) or insulated-gate bipolar transistors (IGBTs). Multiple individually packaged switching devices may be positioned adjacent to each other or, optionally, a switching module may include multiple switching devices within a single housing to provide a reduced cost and more compact footprint for the switching devices. Each switching device is connected to one of the coils to supply power to the coil. The processing, unit generates control signals for each switching device to activate or deactivate the switching device and, in turn, the coil.
Typically, control of a linear drive system has been based largely on control methods employed in a rotational drive system. A rotational drive system sequentially activates coils in a stator to cause rotation of a rotor. A rotational drive system similarly includes switching devices connected to each winding on the stator. Further, a single power module often includes all of the switching devices to control the windings because activation in a rotational drive system is continually repeated around the stator. A linear drive system is similar to “unrolling” the rotational drive system. Stator windings are laid sequentially as the coils in the linear drive system and windings or permanent magnets are, mounted on the movers to serve as the rotor windings or permanent magnets that would be located in the rotor. Unlike a rotational drive system, however, multiple “stators” must be unrolled and placed along the length of travel of the linear drive system. The processor controls, operation of the mover along one “stator” section at a time, passing the rotor between unrolled stator sections.
In order to utilize control strategies employed for rotational drive systems, linear drive systems typically connect adjacent switching devices to adjacent coils along the linear drive system. If a switching module is utilized, each of the switching devices within one module are similarly connected to adjacent coils such that the switching devices control an equivalent to one stator section in a rotational drive system. As previously indicated, however, adjacent coils typically have similar current levels. Thus, at segments along the track that require an elevated current supplied to the coils, the adjacent switching devices or those devices within one switching module all supply that elevated current to the coils. The elevated current within the switching devices causes localized heating within the power converter for the linear drive system. Further, the localized heating of the switching devices may often be the limiting factor in the capacity of the linear drive system. Thus, additional heat removal techniques such as larger heat sinks and/or air or liquid cooling of the switching devices may be required.
Thus, it would be desirable to provide a power converter for a linear drive system having improved control of the switching devices to reduce the effects of localized heating within the power converter.